The present invention relates to microwave-assisted chemical processes and in particular relates to microwave-assisted chemical synthesis, carried out in automated, controlled-flow fashion, using heterogeneous and high-viscosity compositions and while providing for high pressure reactions.
The use of microwaves to provide energy to initiate, drive or control chemical reactions is well-established. Although conceptually, microwaves can be used to affect a wide variety of chemical reactions, the commercial use of microwave assisted chemistry initially grew most rapidly in techniques such as loss-on-drying (gravimetric) moisture analysis and digestion reactions that provided the foundation for content analysis. Indeed, such reactions still represent a major part of the instrument market for microwave assisted chemistry. In particular, gravimetric analysis and digestion can often be carried out in rather robust fashion, for which the longer wavelengths and broad control parameters of microwaves are well-suited.
More recently, interest has greatly increased in using microwaves to drive more sensitive reactions, particularly organic synthesis, and to do so on the smaller scale—and thus more highly controlled basis—that is preferred (or necessary) in many research oriented applications. Both the speed and nature of microwaves offer certain advantages. Because microwaves react immediately with polar and semi-polar materials, they help avoid the lag time inherent in other forms of energy transfer such as conduction or convection heating. Thus, they offer a time advantage for many research schemes including those broadly referred to as “combinatorial chemistry.” Just as importantly, however, electromagnetic radiation in the microwave frequencies can affect individual molecules (and thus compositions) somewhat differently—and thus potentially more favorably—than convention or conduction heating. Some of these advantages are explained in more detail in Hayes, Microwave Synthesis-Chemistry at the Speed of Light, 2002 CEM Publishing (ISBN 0-9722229-0-1).
As an additional factor, single mode cavity microwave instruments have become commercially available that are well-suited for controlled application of microwave radiation to small samples. These include the VOYAGER®, DISCOVER® and EXPLORER® instruments available from CEM Corporation, Matthews, N.C., for which more formal descriptions are set forth (but not limited to) U.S. Published Patent Applications Nos. 20030089706 and 20020117498, U.S. Pat. Nos. 6,867,400; 6,744,024; 6,607,920; and 6,521,876, and pending unpublished application Ser. No. 10/249,011 filed Mar. 10, 2003 and Ser. No. 10/126,838 filed Apr. 19, 2002. The contents of all of these are incorporated entirely herein by reference.
Several of these devices are batch-type devices; i.e. microwaves are applied to a fixed sample in a closed reaction vessel (or a set of fixed samples in several respective separate vessels. When an additional reaction is to be carried out, a new set of starting materials are placed in a new reaction cell which is placed in turn in the microwave cavity. Although the use of individual vessels can be automated, for the most part the reactions must be carried out in batch fashion.
For some commercial applications, however, a constant or continuous reaction scheme—i.e. exposing a continuous stream of reactants to the microwaves and producing a continuous stream of product, without intermittent manipulation of a series of vessels—is attractive or necessary option. The present generation of instrument suitable for this type of reaction is exemplified by CEM's VOYAGER® instrument (e.g. U.S. Pat. No. 6,867,400). This type of instrument is broadly exemplified by an appropriate source of liquid starting materials, suitable fluid pumps (with those for high pressure liquid chromatography, “HPLC” being exemplary), and a flow path that carries the reactants through a microwave field for a time sufficient for a desired reaction to take place. The time spent in the microwave field is, however, dictated by the diameter and length of the flow path taken together with the flow rate of the reactants. Because practical considerations tend to limit the length of the flow path that can be conveniently placed in a commercial instrument, the time spent in the microwave field is also limited.
Although the latest generation of both the batch and continuous systems offer significant advantages for chemical synthesis, each includes characteristics that preclude it from handling certain types of reactions. The batch systems can handle high pressure and heterogeneous starting materials, but cannot offer continuous operation from a reactant source. The flow-through systems can use reactants and generate products on a continuous basis, but generally cannot handle (because of pumping or flow considerations) heterogeneous starting materials or high viscosity fluids, or do so at high pressures. For example, conventional HPLC pumps cannot handle higher viscosity liquids or any solids whatsoever. Even if pumps that can handle solids are incorporated, however, the available flow rates raise specific problems in microwave assisted chemistry. Higher flow rates help move solids through the instrument but reduce the available time spent in the microwave field. Lower flow rates will (mathematically at least) increase residence time in the microwave field, but tend to encourage heterogeneous mixtures (typically liquid reactants and solvents in combination with solid-phase catalysts or solid-supported reagents) to separate into their respective phases before reacting properly or, in severe cases, blocking the flow path and rendering the instrument temporarily or permanently unusable.
Flow-through devices also lack a stirring capability, which can be particularly important for heterogeneous mixtures. Furthermore, precise temperature control (as opposed to consistent application of microwave radiation) is different or impossible in flow-through systems. Additionally, many prior flow-through systems require multimode cavities or otherwise operate in multimode fashion. Finally, conventional flow-through systems can often handle homogeneous liquids at high pressure or heterogeneous mixtures at low pressures, but cannot provide a continuous flow reaction system for carrying out high pressure reactions on heterogeneous materials. Because higher pressures (e.g. up to 250 psi or more) are advantageous to or necessary for certain reaction schemes, the ability to carry them out on an automated or flow through basis presents a function disadvantage.
Prior descriptions of proposed (or actual) flow-though or continuous devices tend to reflect—even if by silence—these limitations.
For example, U.S. Pat. No. 5,387,397 to Strauss discloses a flow-through microwave instrument that can nominally provide “a continuous and pressurized feed of liquid or slurry to and through a microwave heating zone” (column 2, lines 46–47). Of the approximately 27 actual examples included in the '397 patent, however, only two refer explicitly to the use of a heterogeneous mixture. In particular, the preparation of 4-(1-cyclohex-1-enyl) morpholine at column 11, line 62 uses a finely ground starting material in a solvent, and the preparation of phenyl vinyl ketone at column 12 line 16 describes a suspension of 5 grams of starting material in 400 milliliters of water. Other examples may create heterogeneous mixtures, but if so, Strauss does not appear to focus upon them.
Stated differently, the heterogeneous mixtures described by Strauss are in the neighborhood of about one percent by weight of the otherwise liquid volume being pumped. Furthermore, although Strauss refers to pressure control, it is in the nature of a continuous flow system and does not provide for extended residence times.
Katschnig U.S. Pat. No. 5,403,564 describes a microwave system for thermal decontamination of “pumpable or pourable” material, but essentially operates at between about one and two atmospheres.
Knapp U.S. Pat. No. 5,672,316 describes a flow-through system in which higher pressure is equilibrated by placing a flow path within a pressure-containing vessel while leaving one end of the flow path open to the interior of the vessel and a reservoir of liquid in the vessel to thereby cause the pressure on the inside and the outside of the flow path to be identical.
Haswell U.S. Pat. No. 5,215,715, which is commonly assigned with the present invention, describes a flow-through system in which samples to be digested are moved through a microwave cavity as discreet slugs at pressures of between about 30 and 120 pounds per square inch (PSI). The Haswell instrument is primarily for digestion rather than chemical synthesis as indicated by the nature of the flow-through system and the manner in which the slug and solvent are pumped through it.
Renoe U.S. Pat. No. 5,420,039, which is also commonly assigned with the present invention, describes a flow-through system in which water is pumped through at high pressure, but an ordinary sample is carried by the water rather than being pressurized. In particular, pressure is controlled in the 039 patent for the purpose of keeping gasses dissolved in a liquid sample so that the liquid sample can be consistently evaluated using a capacitance detection system.
Accordingly, in spite of the advantages and designs for microwave-assisted instruments, there remains a need for an instrument that can operate in automated, semi-continuous fashion, that can handle highly viscous liquids and heterogeneous mixtures, can do so at high pressure, and with microwave assistance.